This inventions relates to power supply systems, and more particularly to an AC power supply system exhibiting extremely low ground current leakage, specifically in the order of 5 microamperes or less (at 110 volts).
The present invention is ideally suited for any application where extremely low leakage current as well as highly filtered, transient suppressed AC power is required. A major field of application of this invention is in medical instrumentation, used intraoperatively.
The invention utilizes a conventional twin T filter, available from a number of manufacturers. Such a filter provides highly effective filtering, but is subject to significant leakage to ground when connected in the normal manner, which includes grounding the electrical center of the filter and/or the filter case to system ground.
In the present invention, transformer and filter structures are isolated from ground, and the electrical center of the filter structure is connected to a center tap on the secondary winding of the transformer structure. Isolation from system ground is advantageously achieved by mounting the transformer and filter structures on a non-conductive plate that is electrically isolated from system ground.
By this technique, leakage currents to ground are reduced significantly, and in a typical application involving a 110 volt power supply system, ground leakage current is reduced to one-half that specified under Underwriter Laboratory Standard No. 544 for intraoperative use.